My understanding is that with books purchased from Amazon, there is a specific paper edition associated with the Kindle edition, from which actual page numbers are drawn. The source is listed by ISBN on the Kindle edition product page.kiazishiru said:
"Page numbers" shown on books purchased from other sellers are not necessarily actual page numbers, from some specific paper edition, but have been added based on an approximation algorithm for reader convenience. For books with no page numbers you can do a similar thing with Calibre, I think.
But if your goal is to reference specific edition page numbers for citation purposes -- or just to keep up with a book club -- the pages won't necessarily match up with the paper books everyone else uses. For a book club reading, it's probably close enough, but it's not something that would likely be allowed for citations. Which is probably not an issue for most readers.
As I said, my experience is that most trade pubbed books I have purchased from Amazon have page numbers. Many independently published titles do as well, but not all. I haven't searched all 1000 books in my Kindle library, but the ones I've come upon without page numbers are, invariably, independently published. Again, that's MY experience.
